A. Field of the Invention
This invention relates generally to capacitor discharge ignition systems and more particularly to an ignition control arrangement for engine speed control.
B. Description of the Prior Art
Various capacitor discharge systems of the prior art have been developed to provide a breakerless ignition system for the control of an engine whereby an appropriately timed signal is supplied to the primary winding of an ignition coil to induce a high voltage in a secondary winding of the ignition coil to fire a spark plug or spark plugs associated with the engine.
The capacitor discharge systems of the prior art utilize various circuit arrangements and coil arrangements including triggering and charging coils in combination with a rotating permanent magnet on the engine flywheel to induce current in a charging coil, charge a capacitor with the induced current and discharge the capacitor into the primary winding of an ignition coil. Such arrangements are shown for example in U.S. Pat. No. 3,941,111 which issued to T. F. Carmichael on Mar. 2, 1976; U.S. Pat. No. 4,056,088 which issued to T. F. Carmichael on Nov. 1, 1977; U.S. Pat. No. 4,036,201 which issued to B. O. Burson on July 19, 1977; U.S. Pat. No. 3,500,809 which issued to G. Hohne on Mar. 17, 1970; U.S. Pat. No. 3,851,636 which issued to F. H. Just et al on Dec. 3, 1974; U.S. Pat. No. 3,703,889 which issued to B. Bodig et al on Nov. 29, 1972; U.S. Pat. No. 3,704,701 which issued to G. Streuber et al on Dec. 5, 1972; U.S. Pat. No. 3,484,677 which issued to M. J. Piteo on Dec. 16, 1969; and U.S. Pat. No. 3,722,488 which issued to T. E. Swift et al on Mar. 27, 1973.
The capacitor discharge ignition (CDI) systems of the prior art have also provided arrangements to advance the timing at higher engine speeds for improved engine performance and to provide proper ignition timing over a wide range of engine speeds to enable proper ignition at low RPM and to prevent excessive retarding of the engine timing or spark at very high engine RPM's.
For example in U.S. Pat. No. 3,941,111, a CDI system is disclosed wherein a capacitor is charged through a charge coil during a first portion of a pulse of one polarity and an SCR is triggered during a second portion of the same pulse to discharge the capacitor into the primary winding of an ignition coil. In FIG. 2 of this patent, the primary winding controls the SCR by means of a resistor-capacitor network. Specifically, a resistor 50 is connected between one end of a primary winding and the control gate electrode of the SCR and a capacitor is connected between the control electrode of the SCR and the other end of the primary winding. Further, an additional resistor 48 is connected across the capacitor 38 to prevent the spark from retarding at very high engine RPM.
In U.S. Pat. No. 4,056,088 and referring specifically to FIGS. 3 and 4, the CDI system disclosed therein provides an advance in ignition timing of approximately 8.degree. to 10.degree. at higher engine speeds. At column 5, lines 38 through 59, it is stated that the effect is highly desirable although the exact cause of the advance is not fully understood. The specification states that it is thought to be a part of the function of the spacing of the legs 22A and 22B of the stator core 22 relative to the spacing of the poles 18 and 20 of the permanent magnet structure carried by the flywheel.
In U.S. Pat. No. 3,500,809 there is disclosed a circuit arrangement to provide an automatic advancement of the timing or spark at higher engine speeds. This is accomplished by the control voltage to the SCR 7 rising faster with increased speed of the engine. A resistor-capacitor network or a shorting coil is provided to control this effect with varying engine speeds.
The automatic timing advance arrangement disclosed in U.S. Pat. No. 3,722,488 utilizes a trigger waveform including a first high speed trigger peak and a second low speed trigger peak. Specifically, the trigger waveform with two peaks is generated in a trigger coil; the trigger waveform including two distinct positive peaks with an appropriate timing relationship. The first trigger peak at low engine speeds is insufficient to trigger the control circuitry to discharge a charging capacitor. At low speeds the second, higher peak triggers the system at a point later in time and thus retarded with respect to the occurrence of the first peak. At high engine speeds, the first, lower level peak is sufficient to trigger the system and thus occurs at an advanced time with respect to the second peak. As the engine speed increases to a predetermined speed, the timing control is shifted from the second peak to the first peak with a resultant increase or advance in timing.
The capacitor discharge systems discussed hereinbefore are generally suitable for their intended use to advance the ignition timing at high engine speeds. However, in many applications of capacitor discharge systems for the ignition timing control of small engine such as in chain saws, plastic cord grass cutters and the like, it would be desirable to provide engine speed control via the capacitor discharge system to prevent excessively high engine speeds under no load conditions or other situations in which the speed of the engine increases sharply beyond desirable operating speeds. The capacitor discharge system of the prior art provide a relative measure of ignition timing retard at high engine speeds as evidenced by the prior art arrangements to overcome ignition timing retard and to provide ignition timing advance at higher engine speeds. However, the arrangements of the prior art do not provide a significant predetermined ignition timing retard characteristics at high engine speeds to achieve engine speed control and to prevent engine damage due to excessively high speeds.
For example, the capacitor ignition system of U.S. Pat. No. 4,036,201 exhibits several degrees of ignition timing retard at higher engine speeds. Attempts to increase the ignition timing retard by increasing the inductance of the primary winding of the ignition transformer results in a lowering of the ignition voltage or an impractical size of the ignition transformer.